I understand that the blockchain is what keeps track of who has what coins and makes sure that everything is correct and secure, but why do I have to download it? Isn't that where the miners come in? Don't they keep track of the blockchain?

3 Answers
3

Certainly miners, and many other users, keep track of the block chain.

But if you don't download and verify the block chain yourself, what will you do when you want to know whether a transaction is valid? You could ask a miner, but what if they decide to lie to you? You would have to trust them to tell you the truth.

Part of the Bitcoin security model is that you don't have to trust anybody. When you verify the block chain for yourself, nobody can trick you into believing that an invalid transaction is valid, or vice versa.

Ok great, and maybe I don't understand what mining is exactly, but me verifying the block chain is not mining? If I understand correctly, I am not helping verifying the block chain, but rather am getting the blockchain that has been verified by the entire BTC network?
– Hurricane DevelopmentJul 11 '15 at 2:51

No, you're not mining. It's not really accurate to say that the purpose of mining is "verifying" the block chain, i.e. making sure all transactions are valid, because as we see, everyone can do that for themselves. The purpose of mining is to ensure that valid transactions cannot later be undone (made invalid). For more information, start with bitcoin.stackexchange.com/questions/148/what-exactly-is-mining
– Nate EldredgeJul 11 '15 at 3:26

Bitcoin is trustless, so you need to validate the transaction yourself. The miner are rewarded for protecting the network by working on special hash. A miner that would release a block with an invalid transaction would be rejected by all nodes and would not work... because everyone is validating everything.

The whole blockchain is not required to validate, in fact you need only unspent outputs. Right now there is no way to get that beside download the whole blockchain.